The present invention relates to the consolidation of porous, moist webs by pressing and drying, and more particularly to new and improved methods and apparatus for consolidating thin fibrous webs, especially soft papers such as toilet and facial tissues, paper towels, and thin printing papers, for example, although it is not limited to such applications.
Moist paper webs are commonly dried by pressing them against heated rolls. For example, in drying with a so-called Yankee dryer, the web is firmly pressed against a steam heated cylinder of large diameter, which must have considerable thickness to withstand the internal pressure and external load. Usually, a dryer or ventilating hood surrounds part of the cylinder periphery to assist in drying the web. In order to meet the high capacity demands of modern tissue machines, cylinders of very large diameter have been used, yet more than 60% of the drying energy comes from the hood. Such machinery is large, costly to build and operate, and occupies considerable space in the plant.
Multicylinder machines have also been used in which the web is first pressed between rolls, together with one or two felts and possibly accompanying wires. The web is usually taken through two to four press nips in order to remove as much water as possible by mechanical means before final consolidation is effected by drying with heat. This is done by pressing the web against steam heated cylinders by web tensioning or with the aid of a felt or wire adjusted to apply a very light load. Drying then takes place partly while the web is being heated on the cylinders and partly by evaporation of water from the web in the open and preferably ventilated draws between the cylinders. The contact pressure between the web and the cylinders, however, must be kept very low if damage to the web is to be avoided, and this limits the heat transfer from the cylinders to the web. Multicylinder machines are therefore very large and long in relation to the production capacity allowed by the dryer section.
It has also been proposed to dry a web of wet fibrous material by running it over externally heated rollers to generate a vapor layer on which the web floats, as disclosed in Canadian Pat. No. 1,013,316. Since the web does not come into direct contact with the heated roller, heat transfer between the web and the rollers is limited.